Problem: The geometric sequence $(a_i)$ is defined by the formula: $a_1 = -9$ $a_i = -\dfrac{1}{3}a_{i-1}$ What is $a_{5}$, the fifth term in the sequence?
Explanation: From the given formula, we can see that the first term of the sequence is $-9$ and the common ratio is $-\dfrac{1}{3}$ To find the fifth term, we can rewrite the given recurrence as an explicit formula. The general form for a geometric sequence is $a_i = a_1 r^{i - 1}$ . In this case, we have $a_i = -9 \left(-\dfrac{1}{3}\right)^{i - 1}$ To find $a_{5}$ , we can simply substitute $i = 5$ into the formula. Therefore, the fifth term is equal to $a_{5} = -9 \left(-\dfrac{1}{3}\right)^{5 - 1} = -\dfrac{1}{9}$.